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    <title>Choropleth map with hover effect in d3.js</title>
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        <h1>Choropleth map with hover effect in d3.js</h1>
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        <br /><br />
        <p style="max-width: 700px; margin: auto">
          It is a very common practice to add hover effects on
          <a href="../choropleth.html">choropleth maps</a> with d3.js. Here, the
          hovered regions has its opacity increased when all other regions are
          made more transparent. This example works with d3.js
          <code>v4</code> and <code>v6</code>
        </p>
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          href="../choropleth.html"
          >Choropleth map section</a
        >
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            <div style="position: -webkit-sticky; position: sticky; top: 120px">
              <div class="bg-light" id="result"></div>
              <br />
              <div>
                <h5>Steps:</h5>
                <ul>
                  <br />
                  <li>
                    First of all, see the
                    <a href="../backgroundmap.html">background map</a> section
                    to understand the basics of mapping with d3, and the
                    <a href="choropleth_basic.html">most basic choropleth</a>
                    map post.
                  </li>
                  <br />
                  <li>
                    Map shows the world population per country in 2005. Data
                    comes from
                    <a
                      href="https://www.r-graph-gallery.com/183-choropleth-map-with-leaflet/"
                      >here</a
                    >, stored
                    <a
                      href="https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/world_population.csv"
                      >here</a
                    >
                  </li>
                  <br />
                  <li>
                    Each country is actually a <code>path</code>. Two functions
                    are triggered when the mouse hover/leave the country:
                    <code>mouseOver</code> and <code>mouseLeave</code>
                  </li>
                  <br />
                  <li>
                    Those two functions basically modify the
                    <code>opacity</code> and the <code>stroke</code> of paths.
                  </li>
                </ul>
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            <!-- ========= show html code ============== -->
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              &larr; Edit me!
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            <pre
              class="language-html d-none d3v4-chunk"
            ><code id="code-html-v4" contenteditable="true"><xmp><!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js -->
<script src="http://d3js.org/d3.v4.js"></script>
<script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>
<script src="https://d3js.org/d3-geo-projection.v2.min.js"></script>

<!-- Create an element where the map will take place -->
<svg id="my_dataviz" width="400" height="300"></svg>

</xmp></code></pre>

            <pre
              class="language-html d-none d3v6-chunk"
            ><code id="code-html-v6" contenteditable="true"><xmp><!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js -->
<script src="https://d3js.org/d3.v6.js"></script>

<!-- Create an element where the map will take place -->
<svg id="my_dataviz" width="400" height="300"></svg>

</xmp></code></pre>
            <!-- ==================================== -->

            <!-- ========= show JS code v4 ==============  -->
            <pre
              class="language-js d-none d3v4-chunk"
            ><code id="code-js-v4" contenteditable="true"><xmp><script>

// The svg
var svg = d3.select("svg"),
  width = +svg.attr("width"),
  height = +svg.attr("height");

// Map and projection
var path = d3.geoPath();
var projection = d3.geoMercator()
  .scale(70)
  .center([0,20])
  .translate([width / 2, height / 2]);

// Data and color scale
var data = d3.map();
var colorScale = d3.scaleThreshold()
  .domain([100000, 1000000, 10000000, 30000000, 100000000, 500000000])
  .range(d3.schemeBlues[7]);

// Load external data and boot
d3.queue()
  .defer(d3.json, "https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/world.geojson")
  .defer(d3.csv, "https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/world_population.csv", function(d) { data.set(d.code, +d.pop); })
  .await(ready);

function ready(error, topo) {

  let mouseOver = function(d) {
    d3.selectAll(".Country")
      .transition()
      .duration(200)
      .style("opacity", .5)
    d3.select(this)
      .transition()
      .duration(200)
      .style("opacity", 1)
      .style("stroke", "black")
  }

  let mouseLeave = function(d) {
    d3.selectAll(".Country")
      .transition()
      .duration(200)
      .style("opacity", .8)
    d3.select(this)
      .transition()
      .duration(200)
      .style("stroke", "transparent")
  }

  // Draw the map
  svg.append("g")
    .selectAll("path")
    .data(topo.features)
    .enter()
    .append("path")
      // draw each country
      .attr("d", d3.geoPath()
        .projection(projection)
      )
      // set the color of each country
      .attr("fill", function (d) {
        d.total = data.get(d.id) || 0;
        return colorScale(d.total);
      })
      .style("stroke", "transparent")
      .attr("class", function(d){ return "Country" } )
      .style("opacity", .8)
      .on("mouseover", mouseOver )
      .on("mouseleave", mouseLeave )
    }

</script></xmp></code></pre>
            <!-- ==================================== -->

            <!-- ========= show JS code v6 ==============  -->
            <pre
              class="language-js d-none d3v6-chunk"
            ><code id="code-js-v6" contenteditable="true"><xmp><script>

// The svg
const svg = d3.select("svg"),
  width = +svg.attr("width"),
  height = +svg.attr("height");

// Map and projection
const path = d3.geoPath();
const projection = d3.geoMercator()
  .scale(70)
  .center([0,20])
  .translate([width / 2, height / 2]);

// Data and color scale
const data = new Map();
const colorScale = d3.scaleThreshold()
  .domain([100000, 1000000, 10000000, 30000000, 100000000, 500000000])
  .range(d3.schemeBlues[7]);

// Load external data and boot
Promise.all([
d3.json("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/world.geojson"),
d3.csv("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/world_population.csv", function(d) {
    data.set(d.code, +d.pop)
})]).then(function(loadData){
    let topo = loadData[0]

    let mouseOver = function(d) {
    d3.selectAll(".Country")
      .transition()
      .duration(200)
      .style("opacity", .5)
    d3.select(this)
      .transition()
      .duration(200)
      .style("opacity", 1)
      .style("stroke", "black")
  }

  let mouseLeave = function(d) {
    d3.selectAll(".Country")
      .transition()
      .duration(200)
      .style("opacity", .8)
    d3.select(this)
      .transition()
      .duration(200)
      .style("stroke", "transparent")
  }

  // Draw the map
  svg.append("g")
    .selectAll("path")
    .data(topo.features)
    .enter()
    .append("path")
      // draw each country
      .attr("d", d3.geoPath()
        .projection(projection)
      )
      // set the color of each country
      .attr("fill", function (d) {
        d.total = data.get(d.id) || 0;
        return colorScale(d.total);
      })
      .style("stroke", "transparent")
      .attr("class", function(d){ return "Country" } )
      .style("opacity", .8)
      .on("mouseover", mouseOver )
      .on("mouseleave", mouseLeave )

})

</script></xmp></code></pre>
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            <h3 class="text-uppercase">Related blocks &rarr;</h3>
          </div>
          <div class="col-lg-5">
            <ul>
              <li>
                <i>World Choropleth - </i
                ><a
                  href="http://bl.ocks.org/palewire/d2906de347a160f38bc0b7ca57721328"
                  >link</a
                >
              </li>
              <br />
              <li>
                <i>Projection Transitions - </i
                ><a
                  href="https://bl.ocks.org/alexmacy/6700d44240d2b6d3ec9767a5a5854e42"
                  >link</a
                >
              </li>
              <br />
              <li>
                <i>Choropleth section - </i
                ><a href="../choropleth.html">link</a>
              </li>
            </ul>
          </div>
        </div>
      </div>
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